Integrate circuit with nanowires

ABSTRACT

A method includes providing a substrate having a metal-oxide-semiconductor (MOS) region. The MOS region includes first gate, source, and drain regions for a first device, and second gate, source, and drain regions for a second device. The first gate region has a first length. The second gate region has a second length different from the first length. The method further includes forming first and second fins in the first and second gate regions, forming first and second semiconductor layer stacks over the first and second fins, and performing a thermal oxidation process to the first and second semiconductor layer stacks, thereby forming first and second nanowire sets in the first and second gate regions respectively. The first and second nanowire sets are wrapped by respective semiconductor oxide layers. The first nanowire set has a first diameter. The second nanowire set has a second diameter different from the first diameter.

This is a continuation application of U.S. patent application Ser. No.14/477,588, entitled “Integrated Circuit with Nanowires,” which is acontinuation application of U.S. patent application Ser. No. 14/010,196,now issued U.S. Pat. No. 8,872,161, both of which are hereinincorporated by reference in their entirety.

BACKGROUND

The integrated circuit (IC) industry has experienced exponential growth.Technological advances in IC materials and design have producedgenerations of ICs where each generation has smaller and more complexcircuits than the previous generation. In the course of IC evolution,functional density (i.e., the number of interconnected devices per chiparea) has generally increased while geometry size (i.e., the smallestcomponent (or line) that can be created using a fabrication process) hasdecreased. This scaling down process generally provides benefits byincreasing production efficiency and lowering associated costs.

Such scaling down has also increased the complexity of processing andmanufacturing ICs and, for these advances to be realized, similardevelopments in IC processing and manufacturing are needed. For example,a three dimensional transistor, such as a semiconductor device withnanowires, has been introduced to replace a planar transistor. It isdesired to have improvements in this area.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure is best understood from the following detaileddescription when read with the accompanying figures. It is emphasizedthat, in accordance with the standard practice in the industry, variousfeatures are not drawn to scale and are used for illustration purposesonly. In fact, the dimensions of the various features may be arbitrarilyincreased or reduced for clarity of discussion.

FIG. 1A is a diagrammatic perspective view of an integrated circuit (IC)with nanowire according to an embodiment of the present disclosure.

FIG. 1B is a cross-sectional view of an IC with nanowires along line A-Ain FIG. 1A.

FIG. 1C is a cross-sectional view of an IC with nanowires along line B-Bin FIG. 1A. The line B-B is perpendicular to the line A-A.

FIG. 1D is a diagrammatic perspective view of an IC with nanowireaccording to an embodiment of the present disclosure.

FIG. 2A is a diagrammatic perspective view of an IC with nanowireaccording to an embodiment of the present disclosure.

FIGS. 2B-2C and 3A-3B are cross-sectional views of an example IC withnanowire along line A-A in FIG. 2A.

FIG. 4 is a top view of an example IC with nanowire sets.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

The present disclosure is directed to, but not otherwise limited to, acomplementary metal-oxide-semiconductor (CMOS) device comprising aP-type metal-oxide-semiconductor (PMOS) device and an N-typemetal-oxide-semiconductor (NMOS) device. The following disclosure willcontinue with a CMOS device example to illustrate various embodiments ofthe present invention. It is understood, however, that the presentdisclosure should not be limited to a particular type of device, exceptas specifically claimed.

FIG. 1A is a side-perspective view of a IC 100 according to anembodiment of the present disclosure. FIGS. 1B and 1C are cross-sectionviews of the IC 100 along line A-A and B-B, respectively, of FIG. 1A.The line B-B is perpendicular to the direction of the line of A-A. FIG.1D is a side-perspective view of the IC 100 according to anotherembodiment of the present disclosure. The remaining figures provide sideand cross-sectional views of the IC 100, according to various stages offabrication.

Referring to FIGS. 1A-1C, the IC 100 may be a part of a largerintegrated circuit (IC) with a plurality of different devices, regions,and areas, such as a p-type MOS (PMOS) and/or an N-type MOS (NMOS) inand on a substrate 210. As shown in the figure, the substrate 210includes a source/drain region 212 and a gate region 214 having a lengthL, which may vary throughout the device. For example, the gate regioncan have a first length L₁ at one location, and a second length L₂ atanother. In the present embodiment, the second length L₂ is larger than20 nm, which is more than 20% longer than the first length L₁. The gateregion 214 having the first length L₁ is referred to as a short channelgate region while the gate region 214 having the second length L₂ isreferred to as a long channel gate region. The source/drain regions 212are separated by the gate region 214.

In the present embodiment, the substrate 210 is a bulk siliconsubstrate. Alternatively, the substrate 210 may include an elementarysemiconductor, such as silicon or germanium in a crystalline structure;a compound semiconductor, such as silicon germanium, silicon carbide,gallium arsenic, gallium phosphide, indium phosphide, indium arsenide,and/or indium antimonide; or combinations thereof. Possible substrates210 also include a semiconductor-on-insulator substrate, such assilicon-on-insulator (SOI), SiGe-On-Insulator (SGOI), Ge-On-Insulatorsubstrates. For example, SOI substrates are fabricated using separationby implantation of oxygen (SIMOX), wafer bonding, and/or other suitablemethods.

Some exemplary substrates 210 also include an insulator layer. Theinsulator layer comprises any suitable material, including siliconoxide, sapphire, and/or combinations thereof. An exemplary insulatorlayer may be a buried oxide layer (BOX). The insulator is formed by anysuitable process, such as implantation (e.g., SIMOX), oxidation,deposition, and/or other suitable process.

The substrate 210 may include various doped regions depending on designrequirements as known in the art. The doped regions may be doped withp-type dopants, such as boron or BF2; n-type dopants, such as phosphorusor arsenic; or combinations thereof. The doped regions may be formeddirectly on the substrate 210, in a P-well structure, in an N-wellstructure, in a dual-well structure, or using a raised structure.

A recessed first fin 220 is formed in the source/drain region 212. Inone embodiment, the recessed first fin 220 is formed by forming a firstfin over the substrate 210 first and recessing the first fin. The firstfin may be formed by any suitable process including various deposition,photolithography, and/or etching processes. In an example, the first finis formed by patterning and etching a portion of the silicon substrate210. In another example, the first fin is formed by patterning andetching a silicon layer deposited overlying an insulator layer (forexample, an upper silicon layer of a silicon-insulator-silicon stack ofan SOI substrate). It is understood that first fin may include multipleparallel fins formed in a similar manner.

Various isolation regions 230 are formed on the substrate 210 to isolateactive regions. For example, the isolation region 230 separates thefirst fins. The isolation region 230 may be formed using traditionalisolation technology, such as shallow trench isolation (STI), to defineand electrically isolate the various regions. The isolation region 230includes silicon oxide, silicon nitride, silicon oxynitride, an air gap,other suitable materials, or combinations thereof. The isolation region230 is formed by any suitable process. As one example, the formation ofan STI includes a photolithography process, etching a trench in thesubstrate (for example, by using a dry etching and/or wet etching), andfilling the trench (for example, by using a chemical vapor depositionprocess) with one or more dielectric materials. The trenches may bepartially filled, as in the present embodiment, where the substrateremaining between trenches forms a fin structure. In some examples, thefilled trench may have a multi-layer structure such as a thermal oxideliner layer filled with silicon nitride or silicon oxide.

Then the first fin is recessed to form the recessed first fin 220. Therecessing process may include dry etching process, wet etching process,and/or combination thereof. The recessing process may also include aselective wet etch or a selective dry etch. A wet etching solutionincludes a tetramethylammonium hydroxide (TMAH), a HF/HNO3/CH3COOHsolution, or other suitable solution. Dry etching processes include abiased plasma etching process that uses a chlorine-based chemistry.Other dry etchant gasses include CF₄, NF₃, SF₆, and He.

A source/drain feature 240 is formed over the recessed first fin 220 inthe source/drain region 212. In one embodiment, a first semiconductormaterial layer is deposited over the recessed first fin 220 by epitaxialgrowing processes to form the source/drain feature 240. The epitaxialprocesses include CVD deposition techniques (e.g., vapor-phase epitaxy(VPE) and/or ultra-high vacuum CVD (UHV-CVD)), molecular beam epitaxy,and/or other suitable processes. The first semiconductor material layersmay include germanium (Ge), silicon (Si), gallium arsenide (GaAs),aluminum gallium arsenide (AlGaAs), silicon germanium (SiGe), galliumarsenide phosphide (GaAsP), or other suitable materials. Thesource/drain features 240 may be in-situ doped during the epi process.For example, the epitaxially grown SiGe source/drain features 240 may bedoped with boron; and the epitaxially grown Si epi source/drain features240 may be doped with carbon to form Si:C source/drain features,phosphorous to form Si:P source/drain features, or both carbon andphosphorous to form SiCP source/drain features. In one embodiment, thesource/drain features 240 are not in-situ doped, an implantation process(i.e., a junction implant process) is performed to dope the source/drainfeatures 240.

Alternatively, an intra-region 216 is formed between two isolationregions 230, as shown in FIG. 1D. In the intra-region 216, eachindividual first fin is removed to form a mesa 218 over the substrate210. A common source/drain feature 240 is formed over the mesa 218 inthe source/drain region 212. In one embodiment, the common source/drainfeature 240 connects directly to each nanowire set 310 in the gateregion 214.

An interlayer dielectric (ILD) layer 250 over the substrate 210,including between the source/drain features 240. The ILD layer 250includes silicon oxide, oxynitride or other suitable materials. The ILDlayer 250 may include a single layer or multiple layers. The ILD layer250 is formed by a suitable technique, such as CVD, ALD and spin-on(SOG). A chemical mechanical polishing (CMP) process may be performed toplanarize the top surface of the ILD layer 250.

In the present embodiments, one or more nanowire sets 310 andhigh-k/metal gates (HK/MG) 320 are formed over the substrate 210 in thegate region 214. Each nanowire set 310 may have a single nanowire ormultiple nanowires. Each nanowire of one nanowire set 310 may connectwith respective source/drain feature 240. In one embodiment, thenanowire set 310 connects with the respective source/drain feature 240directly. The nanowire in the nanowire set 310 may be formed as arod-shape-like and has a diameter d, which will be described moredetails later.

The HK/MG 320 may include an interfacial layer (IL) 322, a HK dielectriclayer 324 and a MG 326. The IL 322 and HK dielectric layer 324 aredisposed over the substrate 210, including conformably wrapping over thenanowire set 310. The HK dielectric layer 324 may include LaO, AlO, ZrO,TiO, Ta2O5, Y2O3, SrTiO3 (STO), BaTiO3 (BTO), BaZrO, HfZrO, HfLaO,HfSiO, LaSiO, AlSiO, HfTaO, HfSiO, (Ba,Sr)TiO3 (BST), Al2O3, HfAlO,Si3N4, oxynitrides (SiON), or other suitable materials. The IL 322 andHK dielectric layer 324 may be deposited by ALD or other suitablemethod.

The MG 326 may include a single layer or multi layers, such as metallayer, liner layer, wetting layer, and adhesion layer. The MG 326 mayinclude Ti, Ag, Al, TiAlN, TaC, TaCN, TaSiN, Mn, Zr, TiN, TaN, Ru, Mo,Al, WN, Cu, W, or any suitable materials. The MG 326 may be formed byALD, PVD, CVD, or other suitable process. The MG 326 may be formedseparately for the NMOS and PMOS with different metal layers.

The following description will be directed to the formation andstructure of the nanowire set 310 in the gate region 214 in processstages, which are earlier than the stage of FIG. 1A-1C. An example ofthe formation and structure of the nanowire set 310 is shown in FIGS.2A-2C.

Referring to FIGS. 2A-2C, a second fin 420 is formed over the substrate210 in the gate region 214. A formation of the second fin 420 is similarin many respects to the first fin 220 discussed above in associationwith FIG. 1A. In one embodiment, the recessed first fin 220 and secondfin 420 are same fins. The isolation region 230 is disposed betweensecond fins 420.

In the present embodiment, the second fins 420 are recessed and asemiconductor layer stack 430 is formed over the recessed second fin420. The semiconductor layer stack 430 may include multiplesemiconductor layers. Each of these semiconductor layers may havesubstantial different thickness to each other. The semiconductor layerstack 430 may include germanium (Ge), silicon (Si), gallium arsenide(GaAs), silicon germanium (SiGe), gallium arsenide phosphide (GaAsP), orother suitable materials. The semiconductor layer stack 430 may bedeposited by epitaxial growing processes, such as CVD, VPE, UHV-CVD,molecular beam epitaxy, and/or other suitable processes.

In one embodiment, in a PMOS unit (as shown in FIG. 2B), thesemiconductor layer stack 430 has (from bottom to top) SiGe(433)/Si(434)/SiGe(433)/Si(434)/SiGe(433)/Si(434) while in NMOS unit (asshown in FIG. 2C) has SiGe (433)/Si (434)/SiGe (433)/Si(434)/SiGe(433).

The semiconductor layer stack 430 may include other suitablecombinations of different semiconductor layers. A chemical CMP processmay be performed to planarize the top surface of the semiconductor layerstack 430 with the isolation region 230.

In the present embodiment, the isolation region 230 may be etched backto form an open spacing to laterally expose at least a portion of thesemiconductor layer stack 430. The etching processes may includeselective wet etch or selective dry etch, such that having an adequateetch selectivity with respect to the semiconductor layers stack 430.Various sizes of open spacing are designed to meet certain devicestructure needs, such as an open spacing for an interconnection contactto be formed later. As an example, a first open spacing s₁ in a firstregion 610 is substantially larger than a second open spacing s₂ in asecond region 620. The second spacing s₂ is substantially larger than athird open spacing s₃ in a second region 630. For Example, in the PMOSunit, s₁ is in a range of 25 nm to 150 nm, s₂ is in a range of 20 nm to50 nm and s₃ is in a range of 20 nm to 35 nm. For another example, inthe NMOS unit, s₁ is in a range of 25 nm to 150 nm, s₂ is in a range of20 nm to 35 nm and s₃ is in a range of 20 nm to 35 nm.

In the PMOS unit, after the semiconductor layer stack 430 beinglaterally exposed, a first thermal oxidation process is performed to theexposed semiconductor material layer stack 430 in the gate region 214.During the thermal oxidation process, at least a portion of eachsemiconductor layers in the semiconductor layer stack 430 is convertedto a semiconductor oxide layer. The thermal oxidation process may beconducted in oxygen ambient, or in a combination of steam ambient andoxygen ambient.

In the NMOS unit, after the semiconductor layer stack 430 beinglaterally exposed, a selective etch process is performed to remove onetype semiconductor layer in the semiconductor layer stack 430 and leaveanother type of semiconductor layer suspend in the gate region 214(supported by the source/drain feature 240). As an example, SiGe layer433 is removed by the selective etch and Si layer 434 is suspended inthe gate region 214. Then a second thermal oxidation process isperformed. The second thermal oxidation process is similar in manyrespects to the first thermal process discussed above. In oneembodiment, the first and second thermal process is one thermal process.

Referring to FIGS. 3A-3B, in the present embodiment, the first/secondthermal oxidation is controlled to convert the exposed semiconductorlayer stack 430 to a designed configuration of a semiconductor oxidelayer stack 530, which has a wire feature 532 in predeterminedsemiconductor oxide layers. As an example, in the PMOS unit, the SiGelayer 433 is converted to a silicon germanium oxide 533 having a Ge wirefeature 532 while the Si layer 434 is fully converted to the siliconoxide layer 534. The Ge wire feature 532 is referred to as Ge nanowire.As another example, in the NMOS unit, the suspended Si layer 434 isconverted to the silicon oxide layer 534 having a Si wire feature 535.The Si wire feature 535 is referred to as Si nanowire.

In present embodiment, a diameter of wire feature 532/535 in the first,second and third regions, 610, 620 and 630 are substantially different.In one embodiment, the wire feature 532/535 formed in the first region610 has a first diameter d₁ which is substantially smaller than a seconddiameter d₂ of the wire feature 532/535 formed in the second region 620.The second diameter d₂ is substantially smaller than a third diameter d₃in the third region 630. In one embodiment, the first diameter d₁ is 10%or smaller than the second diameter d₂. The second diameter d₂ is 10%smaller than the third diameter d₃. For example, in the PMOS unit, d₁ isin a range of 4 nm to 15 nm, d₂ is in a range of 1 nm to 3 nm and d₃ isin a range of 1 nm to 3 nm. For another example, in the NMOS unit, d₁ isin a range of 4 nm to 13 nm, d₂ is in a range of 1 nm to 3 nm and d₃ isin a range of 1 nm to 3 nm.

In present embodiment, the diameter of the wire feature 532/535 in thelong gate region is substantially smaller than the diameter of therespective wire feature 532/535 in the short gate region. In oneembodiment, the diameter of the wire feature 532/535 in the long regionis 20% or smaller than the diameter of the respective wire feature532/535 in the short gate region.

After forming the wire feature 532/535, all layers of the semiconductoroxide stack 530 are removed by a selective etching process and the wirefeatures 532/535 remain in the gate region 214. The wire features532/535 aligning vertically in a same location of the gate region 214are referred to as the nanowire set 310, as shown in FIGS. 1A-1C. TheHK/MG 320 is formed in the gate region 214, including conformablywrapping over the nanowire set 310, as being described in FIGS. 1A-1C.

Referring to FIG. 4, in one embodiment, a first transistor 710 isadjacent to a second transistor 720, each transistor may include amultiple nanowire sets 310. For the sake of description, diameters ofnanowire set 310 located in the very left side, very right side andbetween them, of the transistor 710, are D₁l, D₁r and D₁c respectively.While diameters of nanowire set 310 located in the very left side, veryright side and between them, of the transistor 720, are D₂l, D₂r and D₂crespectively. There may be more than one nanowire set 310 locatedbetween nanowire sets located in very left and right sides. Spacingbetween gate electrode 326 of transistor 710 and 720 is Sg. An outerspacing of the transistors 710 are So(l) and So(r) respectively. Spacingwithin the first transistor 710 is Si. In one embodiment, the firsttransistor 710 includes three nanowire sets 310. Both of So(l) and So(r)is larger than Si, D_(1c) is larger than D₁l and D₁r. In anotherembodiment, In one embodiment, the first transistor 710 include fournanowire sets 310. Both of So(1) and So(r) is larger than Si, D_(1c)(there are two nanowire sets between the very left and right nanowireset) are larger than D₁l and D₁r.

The IC 100 may have various additional features and regions for a CMOSor MOS device, known in the art. For example, variouscontacts/vias/lines and multilayers interconnect features (e.g., metallayers and interlayer dielectrics) are formed over the substrate 210,configured to connect the various features or structures of the IC 100.A multilayer interconnection may include vertical interconnects, such asconventional vias or contacts, and horizontal interconnects, such asmetal lines. The various interconnection features may utilize variousconductive materials including copper, tungsten, aluminum, and/orsilicide, for example, PtSi, CoSi2, NiSi, NiPtSi, WSi2, MoSi2, TaSi2, orother refractory metal silicide. In one example, a damascene and/or dualdamascene process is used to form a copper related multilayerinterconnection structure.

Based on the above, the present disclosure offers an integrated circuitwith nanowire set in a PMOS unit and a NMOS unit. The nanowire set hasone or more nanowire. The nanowire is formed with different diameteraccording to its different environments and locations, such as a size ofan open spacing between adjacent nanowire set, or a gate region length.

The present disclosure provides many different embodiments of anintegrated circuit (IC). The IC includes a substrate having ametal-oxide-semiconductor (MOS) region, first gate, source and drainregions of a first device in the MOS region. The first gate region has afirst length. The IC also includes a first nanowire set disposed in thefirst gate region, the first nanowire set including a nanowire having afirst diameter and connecting to a first feature in the first sourceregion and the first feature in the first drain region. The IC alsoincludes second gate, source and drain regions of a second device in theMOS region. The second gate region has a second length. The IC alsoincludes a second nanowire set disposed in the second gate region, thesecond nanowire set including a nanowire having a second diameter andconnecting to a second feature in the second source region and thesecond feature in the second drain region. If the first length isgreater than the second length, the first diameter is less than thesecond diameter. If the first length is less than the second length, thefirst diameter is greater than the second diameter.

In another embodiment, an integrated circuit includes a substrate havinga metal-oxide-semiconductor (MOS) region, first gate, source and drainregions of a first device in the MOS region having a first gate regionlength. The IC also includes a plurality of first nanowire sets disposedin the first gate region having a different spacing between two adjacentfirst nanowire sets, the first nanowire set including a nanowire havinga first diameter and connecting to a common feature in the first sourceregion and a common feature in the first drain region. A diameter of thefirst nanowire set is different from the first diameter of a differentfirst nanowire set if the different first nanowire set has a differentspacing. The IC also includes second gate, source and drain regions of asecond device in the MOS region having a second gate region length. TheIC also includes a second nanowire set disposed in the second gateregion, the second nanowire set including a nanowire having a seconddiameter and connecting to a feature in the second source region and afeature in the second drain region. If the first length is greater thanthe second length, the first diameter is less than the second diameterand if the first length is less than the second length, the firstdiameter is greater than the second diameter.

In yet another embodiment, an integrated circuit (IC) includes asubstrate having an N-type metal-oxide-semiconductor (NMOS) region and aP-type metal-oxide-semiconductor (PMOS) region, a plurality of gatestructures in the NMOS region and in the PMOS region. A length of thegate structures and a spacing of the gate structures varies between atleast two thereof. A nanowire set is disposed in the each of theplurality of gate structures. A diameter of each nanowire in eachnanowire set corresponds directly with a relative spacing to an adjacentgate structure, and a relative length of the gate structure.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method for integrated circuit (IC)manufacturing, the method comprising: providing a substrate having ametal-oxide-semiconductor (MOS) region, wherein the MOS region includesfirst gate, source, and drain regions for a first device, and secondgate, source, and drain regions for a second device, and wherein thefirst gate region has a first length, and the second gate region has asecond length that is different from the first length; forming a firstfin in the first gate region; forming a second fin in the second gateregion; forming a first semiconductor layer stack over the first fin;forming a second semiconductor layer stack over the second fin; andperforming a thermal oxidation process to the first and secondsemiconductor layer stacks, thereby forming first and second nanowiresets in the first and second gate regions respectively, wherein thefirst and second nanowire sets are wrapped by respective semiconductoroxide layers, wherein the first nanowire set has a first diameter, andwherein the second nanowire set has a second diameter that is differentfrom the first diameter.
 2. The method of claim 1, wherein the first andsecond fins are formed in the same manufacturing process, and the firstand second semiconductor layer stacks are formed in the samemanufacturing process.
 3. The method of claim 1, further comprising:recessing the first fin before the forming of the first semiconductorlayer stack; and recessing the second fin before the forming of thesecond semiconductor layer stack.
 4. The method of claim 1, furthercomprising: forming an isolation feature over the substrate, wherein theisolation feature isolates the first and second fins.
 5. The method ofclaim 1, wherein each of the first and second semiconductor layer stacksincludes a plurality of pairs of a silicon layer over a silicongermanium layer.
 6. The method of claim 5, wherein the second device isa NMOS device, further comprising: selectively removing the silicongermanium layers in the second semiconductor layer stack before theperforming of the thermal oxidation process.
 7. The method of claim 1,further comprising: removing the semiconductor oxide layers, therebyleaving the first and second nanowire sets suspending over the first andsecond fins respectively.
 8. The method of claim 7, further comprising:forming a first gate stack in the first gate region, wherein the firstgate stack includes an interfacial layer conformally wrapping over thefirst nanowire set; and forming a second gate stack in the second gateregion, wherein the second gate stack includes an interfacial layerconformally wrapping over the second nanowire set.
 9. The method ofclaim 1, further comprising: forming a third fin in the second gateregion; and forming a third semiconductor layer stack over the thirdfin, wherein a spacing between the second and third fins is smaller thana spacing between the first and second fins, wherein the performing ofthe thermal oxidation process includes oxidizing the third semiconductorlayer stack to form a third nanowire set over the third fin, and whereinthe third nanowire set has a third diameter that is different from thesecond diameter.
 10. The method of claim 9, wherein the second fin isdisposed between the first and third fins, and wherein the thirddiameter is greater than the second diameter.
 11. A method forintegrated circuit (IC) manufacturing, the method comprising: providinga substrate having a metal-oxide-semiconductor (MOS) region, wherein theMOS region includes first gate, source and drain regions for a firstdevice, and second gate, source and drain regions for a second device;forming first, second, and third fins in the first gate region, whereinthe second fin is disposed between the first and third fins, and aspacing between the second and third fins is smaller than a spacingbetween the third fin and a fin in the second gate region; formingfirst, second, and third semiconductor layer stacks over the first,second, and third fins respectively; and performing a thermal oxidationprocess to the first, second, and third semiconductor layer stacks,thereby forming first, second, and third nanowire sets over the first,second, and third fins respectively, wherein the first, second, andthird nanowire sets are wrapped by respective oxidized semiconductorlayers, wherein the first nanowire set has a first diameter, the secondnanowire set has a second diameter, the third nanowire set has a thirddiameter, and wherein the second diameter is greater than the thirddiameter.
 12. The method of claim 11, further comprising: forming afourth fin in the first gate region, wherein the fourth fin is disposedbetween the first and second fins; and forming a fourth semiconductorlayer stack over the fourth fin, wherein the performing of the thermaloxidation process includes oxidizing the fourth semiconductor layerstack thereby forming a fourth nanowire set over the fourth fin, andwherein the fourth nanowire set has a fourth diameter that is greaterthan the third diameter.
 13. The method of claim 12, wherein the first,second, third, and fourth fins are formed in the same manufacturingprocess; and the first, second, third, and fourth semiconductor layerstacks are formed in the same manufacturing process.
 14. The method ofclaim 11, further comprising: forming an isolation feature between thefirst and second fins and between the second and third fins.
 15. Themethod of claim 11, wherein the first, second, and third semiconductorlayer stacks each include pairs of a first semiconductor material layerover a second semiconductor material layer.
 16. The method of claim 15,further comprising: selectively removing the second semiconductormaterial layers from each of the first, second, and third semiconductorlayer stacks before the performing of the thermal oxidation process. 17.The method of claim 11, further comprising: after the performing of thethermal oxidation process, removing the oxidized semiconductor layers inthe first, second, and third semiconductor layer stacks, thereby leavingthe first, second, and third nanowire sets suspending over the first,second, and third fins respectively.
 18. A method for integrated circuit(IC) manufacturing, the method comprising: providing a substrate havinga metal-oxide-semiconductor (MOS) region, wherein the MOS regionincludes gate, source and drain regions for a MOS device; forming aplurality of semiconductor layer stacks in the gate region; andperforming a thermal oxidation process to the semiconductor layerstacks, thereby forming a plurality of nanowire sets in the gate region,wherein the plurality of nanowire sets includes a left outer nanowireset, a right outer nanowire set, and at least one inner nanowire set,wherein the at least one inner nanowire set has a first diameter, theleft outer nanowire set has a second diameter, the right outer nanowireset has a third diameter, and wherein the first diameter is greater thanthe second diameter.
 19. The method of claim 18, wherein the firstdiameter is greater than the third diameter.
 20. The method of claim 18,further comprising: selectively removing one type of semiconductorlayers in each of the semiconductor layer stacks before the performingof the thermal oxidation process.